%context.getResourceKey() fails with type mismatch inside forEach within unionAll

[johngrimes]

When using %context.getResourceKey() inside a forEach branch within a unionAll, Pathling throws an InvalidUserInputError:

InvalidUserInputError: Function 'getResourceKey', input: Type mismatch: expected ResourceCollection but got Collection

Per the SQL on FHIR spec, %context within a forEach inside a unionAll should refer to the resource root. Pathling appears to resolve %context as a generic Collection rather than a ResourceCollection, which causes getResourceKey() to reject the input.

Minimal reproduction

from pathling import PathlingContext

pc = PathlingContext.create(enable_extensions=True)
ds = pc.read.ndjson("/path/to/ndjson")

view_json = """
{
  "resourceType": "ViewDefinition",
  "resource": "Observation",
  "status": "draft",
  "name": "test",
  "where": [
    {"path": "category.coding.where(system = 'http://terminology.hl7.org/CodeSystem/observation-category' and code = 'vital-signs').exists()"}
  ],
  "select": [
    {
      "unionAll": [
        {
          "column": [
            {"name": "id", "path": "getResourceKey()", "type": "string"},
            {"name": "code_value", "path": "code.coding.first().code", "type": "code"},
            {"name": "value_number", "path": "value.ofType(Quantity).value", "type": "decimal"}
          ]
        },
        {
          "forEach": "component",
          "column": [
            {"name": "id", "path": "%context.getResourceKey()", "type": "string"},
            {"name": "code_value", "path": "code.coding.first().code", "type": "code"},
            {"name": "value_number", "path": "value.ofType(Quantity).value", "type": "decimal"}
          ]
        }
      ]
    }
  ]
}
"""

df = ds.view(json=view_json)  # Throws InvalidUserInputError

Workaround

Move resource-level columns (those using getResourceKey(), subject.getReferenceKey(), etc.) into a separate top-level select outside the unionAll, and keep only the differing columns inside the unionAll branches:

{
  "select": [
    {
      "column": [
        {"name": "id", "path": "getResourceKey()", "type": "string"}
      ]
    },
    {
      "unionAll": [
        {
          "column": [
            {"name": "code_value", "path": "code.coding.first().code", "type": "code"}
          ]
        },
        {
          "forEach": "component",
          "column": [
            {"name": "code_value", "path": "code.coding.first().code", "type": "code"}
          ]
        }
      ]
    }
  ]
}

Environment

• pathling 9.4.0
• PySpark 4.0.2
• Python 3.14.2
• macOS (Darwin 25.3.0, arm64)

[piotrszul]

@johngrimes
Firstly there does not seem to be any mention of '%context' in SQL on FHIR spec (at least I could not find any) so perhaps this should be added to the spec.

I am not sure however that it's correct. For example in:

{
          "forEach": "component",
          "column": [
            {"name": "id", "path": "%context.getResourceKey()", "type": "string"},
            {"name": "code_value", "path": "code.coding.first().code", "type": "code"},
            {"name": "value_number", "path": "value.ofType(Quantity).value", "type": "decimal"}
          ]
      }

The implicit context for evaluation is the current element from the forEach expression and thus value.ofType(Quantity).value is which is evaluated as %context.value.ofType(Quantity).value is evaluated on an element of %resource.component (not on resource).

One can always use: {"name": "id", "path": "%resource.getResourceKey()", "type": "string"} to obtain the key in this scenario.

For unionAll however the context should remain to be the main resource so this needs to be further investigated.

[piotrszul]

Investigation and Test Results

Added test cases in fhirpath/src/test/resources/viewTests/context.json to confirm the behavior of getResourceKey() in unionAll branches. All tests pass.

Findings

1. Bare getResourceKey() in a unionAll branch (no forEach) works correctly. The inputContext remains a ResourceCollection throughout the evaluation chain.

2. %context.getResourceKey() in a unionAll branch (no forEach) also works correctly. Since there is no forEach, %context resolves to the resource itself, so getResourceKey() succeeds.

3. %resource.getResourceKey() inside forEach within unionAll works correctly. This is the correct way to access the resource key from inside a forEach — using %resource rather than %context, since %context inside a forEach resolves to the forEach element (a plain Collection, not a ResourceCollection).

Root Cause of the Originally Reported Failure

Using %context.getResourceKey() inside a forEach fails because %context correctly resolves to the forEach element, which is not a resource. Users should use %resource.getResourceKey() instead when inside a forEach.

[johngrimes]

I think the issue is incorrect as stated.

%context should be the input collection to the expression under evaluation, regardless of what level it is within the view.

Perhaps we need to clarify this within the SQL on FHIR spec, including maybe adding some test cases.